In the GPRS (General Packet Radio Service) extension of the GSM (Global System for Mobile communication) mobile phone system, there are four different coding schemes (CS1-CS4), which are used for data transfer. Coding schemes CS1 to CS3 make use of convolution coding to provide an error correction capability. However, convolution coding is not used in the CS4 scheme. The CS4 scheme is therefore not tolerant of errors. However, the CS4 scheme does employ CRC codes for identifying the existence of errors.
When using CS4, GPRS blocks having one or more bit errors are rejected. Simulations have shown that when the BLER (Block Error Rate) for the CS4 coding scheme is about 10% approximately 40% of the rejected blocks contain only a single error.
Many error patterns, will yield the same syndrome. In the present document, the term “syndrome” means the remainder obtained by modulo-2 division of a received signal by the appropriate generator polynomial. The normal error correction algorithm, called the table lookup decoding algorithm, associates one error pattern with each syndrome. When the syndrome is calculated, the corresponding error pattern can be found in a look-up table and the errors can be corrected. There can be no certainty that the correct error pattern has been found because several error patterns results in the same syndrome. Accordingly, the normal decoding procedure has to be repeated. However, it is well known that table lookup decoding can only be used when there are less than 10 CRC bits because of memory and search time demands.
For CS4 encoding, there are 16 CRC bits and, therefore, table look-up decoding is impractical.